Abstract
Evaluation studies vary in methodological quality. It is essential to develop methodological quality standards for evaluation research that can be understood and easily used by scholars, practitioners, policy makers, the mass media, and systematic reviewers. This article proposes that such standards should be based on statistical conclusion validity, internal validity, construct validity, external validity, and descriptive validity. Methodological quality scales are reviewed, and it is argued that efforts should be made to improve them. Pawson and Tilley's challenge to the Campbell evaluation tradition is also assessed. It is concluded that this challenge does not have any implications for methodological quality standards, because the Campbell tradition already emphasizes the need to study moderators and mediators in evaluation research.